Distribution and Effects of Chemicals in Communities and Ecosystems - Organic Pollutants: An Ecotoxicological Perspective - page 81

More rapid elimination was needed than could be provided by passive diffusion in
order to prevent tissue concentrations reaching toxic levels.
Some models for predicting bioconcentration and biomagnification are presented
in Box 4.1.
4.3 fate of PoLLutantS In SoILS and SedImentS
Regarding soils, a central issue is the persistence and movement of pesticides that are
widely used in agriculture. Many different insecticides, fungicides, herbicides, and
molluscicides are applied to agricultural soils, and there is concern not only about
effects that they may have on nontarget species residing in soil, but also on the pos-
sibility of the chemicals finding their way into adjacent water courses.
Soils are complex associations between living organisms and mineral particles.
Decomposition of organic residues by soil microorganisms generates complex
organic polymers (“humic substances” or simply “soil organic matter”) that bind
together mineral particles to form aggregates that give the soil its structure. Soil
organic matter and clay minerals constitute the colloidal fraction of soil; because
of their small size, they present a large surface area in relation to their volume.
Consequently, they have a large capacity to adsorb the organic pollutants that con-
taminate soil. Within a freely draining soil there are air channels and soil water,
the latter being closely associated with solid surfaces. Depending on their physical
properties, organic compounds become differentially distributed between the three
phases of the soil, soil water, and soil air.
Hydrophobic compounds of high K ow become very strongly adsorbed to soil col-
loids (Chapter 3, Section 3.1), and consequently tend to be immobile and persistent.
OC insecticides such as DDT and dieldrin are good examples of hydrophobic com-
pounds of rather low vapor pressure that have long half-lives, sometimes running
into years, in temperate soils (Chapter 5). Because of their low water solubility and
their refractory nature, the main mechanism of loss from most soils is by volatiliza-
tion. Metabolism is limited by two factors: (1) being tightly bound, they are not freely
available to enzymes of soil organisms, which can degrade them, and (2) they are, at
best, only slowly metabolized by enzyme systems. Because of strong adsorption and
low water solubility, there is little tendency for them to be leached down the soil pro-
file by percolating water. The degree of adsorption, and consequently the persistence
and mobility, is also dependent on soil type. Heavy soils, high in organic matter and/
or clay, adsorb hydrophobic compounds more strongly than light sandy soils, which
are low in organic matter. Strongly lipophilic compounds are most persistent in heavy
soils. When OC insecticides are first incorporated into soil, they are lost relatively
rapidly, mainly due to volatilization, before they become extensively adsorbed to soil
colloids (Figure 4.3). With time, however, most residual OC insecticide becomes
adsorbed, and subsequently there is a period of very slow exponential loss.
In marked contrast to hydrophobic compounds, more polar ones tend to be less
adsorbed and to reach relatively high concentrations in soil water. Phenoxyalkanoic
acids such as 2,4-D and MCPA are good examples (Figure 4.3). Their half-lives in soil
are measured in weeks rather than years, and they are more mobile than OC insec-
ticides in soils. When first applied they are lost only slowly. After a lag period of a